Coring is a signal processing operation in which low-level signal variations, often associated with noise, are removed from a signal to improve its signal-to-noise ratio. For television (TV) signals, for example, noise can be introduced by the transmission path, the RF tuner and amplifiers, the IF amplifiers or external noise sources. Fixed threshold coring, in which low-level signal variations not exceeding a fixed threshold level are removed, is inadequate in a TV receiver because the viewer is more perceptive of noise occurring in a dark scene (i.e. light spots in a dark background) than in a light scene (i.e. dark spots in a light background).
Thus, it is desirable to provide a relatively higher coring threshold for low luminance signal levels (darker scenes) and a relatively lower coring threshold for high luminance signal levels (lighter scenes). An analog circuit arrangement providing this desirable characteristic for TV receivers having analog signal processing is described in U.S. patent application Ser. No. 373,750 entitled DYNAMIC CORING CIRCUIT filed on Apr. 30, 1982 by L. A. Cochran, (now U.S. Pat. No. 4,437,124) which is assigned to the same assignee as is the present invention.
In digital signal processing apparatus, however, a digital signal coring apparatus must perform the coring operation on signals which are digital numbers representing signal levels rather than directly upon the signal levels per se. Thus, digital circuitry must be employed to generate a coring threshold signal, to develop a control signal therefrom under certain conditions, and to develop cored digital signals in response to the control signal.